Remember that something that sounds too good to be true, usually is.

There have been instances where pre-paid cards and vouchers have been sold by criminals engaged in black mailing and other criminal activities, and once you touch that card or voucher, your ip is logged, and if the case is big enough - you will have police at your door, and they confiscate equipment without asking you. This has happened. So stay safe, stay away. And if you chose to interact, do it with an ip that's not your own - and hide your tracks.

After some research it might seem like bestxchanger is legit, but be careful. Who gives money away online for free?